Low defect, high purity crystalline layers grown by selective deposition

ABSTRACT

The purity and perfection of a semiconductor is improved by depositing a patterned mask (12) of a material impervious to impurities of the semiconductor on a surface (14) of a blank (10). When a layer (40) of semiconductor is grown on the mask, the semiconductor will first grow from the surface portions exposed by the openings (16) in the mask (12) and will bridge the connecting portions of the mask to form a continuous layer (40) having improved purity, since only the portions (42) overlying the openings (16) are exposed to defects and impurities. The process can be reiterated and the mask translated to further improve the quality of grown layers.

ORIGIN OF THE INVENTION

The invention described herein was made in the performance of work undera NASA contract and is subject to the provisions of Section 305 of theNational Aeronautics and Space Act of 1958, Public Law 83-568 (72 Stat435; 42 USC 2457).

TECHNICAL FIELD

The present invention relates to techniques for growing low defect, highpurity layers of semiconductors and, more particularly, to the growth oflow defect, high purity silicon on high defect, impure siliconsubstrates.

BACKGROUND ART

Self-sufficiency in energy is a stated national goal. Many of theproposed means to achieve this goal, especially those dependent onfossil fuels, are either environmentally unacceptable or not feasible.

Of the available alternatives, solar energy is the most abundant,inexhaustible single resource available. However, capturing andutilizing solar energy is not simple. Methods are being sought toconvert solar energy to a concentrated, storable form of energy.

One method of converting solar energy to a usable form being prominentlyconsidered is the deployment of large arrays of photovoltaic solarcells, especially in the sunbelt areas such as the southwestern andwestern regions of the United States. The most promising candidate forthe solar cell is a doped silicon material.

Silicon is one of the most plentiful elements in the earth's crust.However, solar cells are presently fabricated from semiconductor-gradesilicon, which has a market price of about $65.00 per kilogram. A numberof current projects are directed to developing the national capabilityto produce low-cost, long-life photovoltaic modules at a rate greaterthan 500 MW per year and at a price of less than $500 per peak kilowattby the year 1986. A drastic reduction in price of material is necessaryto meet these important national objectives. The presence of transitionmetal impurities has been identified as one of the major factors causingdegradation of silicon photovoltaic cells. These impurities have anegative effect on the carrier lifetime and also on the efficiency ofsilicon cells. Both of these factors have been considered asinter-related critical limitations. The minimization of alltransition-metal impurities is a major concern in the production ofsilicon for wide-spread use in solar arrays.

High quality surfaces of crystalline materials absent defects such asdislocations are required for growing layers which are to be fabricatedinto electronic devices having optimum performance. Device performanceusually depends upon the degree of perfection of the substrate materialon which the device is fabricated, because a growing film typicallycontains all the defects of the substrate from which it grows. This hasrequired the use of expensive substrates of high perfection. Manyattempts to produce highly perfect films on imperfect substrates inorder to reduce the cost of good devices have been made, but withoutsubstantial success.

Growing of films on solid state substrates has been practiced as a meansof forming junctions, or for salvaging the substrate. Baliga et al.(U.S. Pat. No. 4,251,299) in FIG. 5 shows non-planar growth of siliconextending beyond etched areas to form a continuous film. However, Baligaet al. use liquid phase epitaxial growth and only want growth to occurin the etched grooves.

There are several references that teach the salvaging of imperfectsemiconductor or doped semiconductor blanks. Endler et al. (U.S. Pat.No. 4,255,206) does grow a film extending into and bridging bevels onmesas. However, he only uses a mask to etch the bevels. There is no maskpresent during liquid or vapor deposition. Mayberry et al. (U.S. Pat.No. 3,559,281) reclaims wafers by stripping the front surface,passivating it and then stripping and polishing the opposite surface andconducting crystal growth thereon.

Barnett et al. (U.S. Pat. No. 3,647,578), Fujii (U.S. Pat. No.3,671,338), and Revesz et al. (U.S. Pat. No. 3,904,453) do grow crystalson a perforated, masked substrate but they restrict growth to the windowopenings. Lawrence (U.S. Pat. No. 3,923,567) reclaims crystals bygettering to concentrate defects at the surface. Rode (U.S. Pat. No.4,050,964) improves smoothness of growth by misorienting the substrate.Lawrence et al. (U.S. Pat. No. 4,062,102) processes reject wafers bystripping and etching to form a pattern for electrodes. MacDonald, Jr.et al. (U.S. Pat. No. 4,131,472) tracks the defects to dies of a mask soas to correct the die openings. Baliga (U.S. Pat. No. 4,128,440) lowersthe temperature to control diffusion. Esseluhn (U.S. Pat. No. 4,160,682)shows an improved slider apparatus. Kotval el at. (U.S. Pat. No.4,124,410) directionally pulls relatively impure silicon to form lowcost, single crystal silicon and Mizrah (U.S. Pat. No. (4,199,379)selectively forms a metal pattern on a crystal by deposition, maskingand etching.

SUMMARY OF THE INVENTION

Because a growing film typically contains all the defects of thesubstrate from which it grows, the obvious solution is to have the filmgrow from a substrate with limited defects. However, low defectsubstrates are expensive. The process of the invention produces highquality, low-defect films or layers on low-quality, inexpensivesubstrates using well established vapor deposition, epitaxial growthtechniques.

This is accomplished in accordance with the invention by masking most ofthe surface of the substrate with a mask having windows exposing aportion of the surface, generally 50% to 95% thereof. The mask isimpervious to diffusion of impurities in the substrate. Epitaxial vapordeposition onto the masked substrate results in film nucleation growingonly in the windows bridging the land area of the mask and forming acontinuous film which completely covers the surface of the substrate.

The number of defects visible to the observer of this masked surface andavailable to the growing film is reduced to the number of defects in thewindows, all other defects are masked. Growth of a crystalline layer onthis new surface, if nucleated in the windows, will contain only thenumber of defects which are present in the windows and defects occurringat the boundaries of the growing film as the film which originated ateach window begins to coalesce. Every reiteration of the masking andgrowth from the vapor will result in a further decrease in the number ofdefects in the surface film. Additionally, the masking will inhibit thediffusion of impurities in the imperfect substrate into the growingfilms. Each reiteration will result in a more perfect film and thicker,more perfect films can be grown with each reiteration.

The substrate can be single crystal or polycrystalline and can be asemiconductor material other than silicon, such as germanium or thewider band gap materials, such as III-IV compounds, suitably galliumarsenide or phosphide. A suitable masking material for silicon issilicon oxide which also can be deposited from the vapor phase. Thesilicon oxide film is then etched to form an array of windows exposingthe underlying substrate surface. Silicon films are then deposited onthe mask, nucleate only in the windows, and grow to form a layer whichcompletely covers the low-quality, low-cost substrate.

The mask has the added advantage of acting as a diffusion barrier tolimit the diffusion of impurities from a less pure substrate into thepurer, grown layer. Reiterations of the masking and the growth processwill insure increasingly more perfect layers. This can be an automatedand an inexpensive process. The equipment necessary to produce the masksand do the epitaxial growth is available in the market place from avariety of sources. The process itself is extremely simple andrelatively low-cost.

These and many other features and attendant advantages of the inventionwill become apparent as the invention becomes better understood byreference to the following detailed description when considered inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a deposition apparatus;

FIG. 2 is a top view of a mask applied to a semiconductor blank;

FIG. 3 is a top view of another type of mask applied to a seimconductorblank;

FIG. 4 is a side view of the coated blank;

FIG. 5 is a side view of a further mask applied to the grown layer; and

FIG. 6 is a side view of the blank showing two grown layers ofsemiconductor according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIGS. 1 and 2, the process of the invention utilizes alow cost, relatively impure blank 10 of semiconductor. The mask 12 of animpurity-impervious material is applied to a surface 14 of the blank.The mask has openings 16 separated by bridges 18 of the mask material.The openings 16 can be in the form of lines 20 in the gradient-like maskshown in FIG. 2, or the openings can be in the form of polygons orcircles 22, as shown in FIG. 3.

The pattern can be applied by applying a continuous layer across thesurface 14 and then selectively removing material for the openings byphotolithography and etching. The pattern of the mask could also beapplied by deposition through a stencil onto the surface, such as asheet of metal containing a pattern of openings. A layer ofsemiconductor is then deposited on the surface of the blank by anysuitable means, such as by vapor or liquid epitaxial depositiontechniques according to procedures well known in the art. Vapordeposition is preferred with silicon.

A blank 10 with patterned mask 12 is then placed in the holder 24 withina deposition chamber 26. The chamber is evacuated by vacuum pump 28.Deposition source 30 is then activated to feed vapors 32 into thechamber. The deposition chamber may be provided with appropriate heatingmeans. As shown in FIG. 4, the vapors react at or near the surface todeposit crystalline semiconductor material which deposits at the surfaceportions 34 within the openings 16. The deposit grows to form a layerwhich fills the openings 16 and forms a connecting portion 36 to thenext opening until a continuous layer 40 is formed. The layer 40 willhave reduced defects throughout, but will have less impurities anddefects within the connecting portions 36, than the portions 42overlying the openings 16.

The coated blank is then removed and a device can be formed on thelayers by doping to form a junction and applying electrodes. The frontsurface may be polished to remove non-planar portions and the grownlayer can be sliced from the blank.

A further enhancement of growth of more perfect and pure crystals can beachieved by reiteration steps in which the mask is displaced, so thatsolid portions overlay the open areas of the previous mask. Referringnow to FIGS. 5 and 6, the coated blank with grown layer 40 is thencovered with a mask 50 having openings 52 overlying the portions 36 andhaving bridges 54 covering the portions 42. Vapor deposition results ingrowth of a layer 56 which has fewer defects than if only mask 12 hadbeen utilized. This process can be repeated as many times as necessaryto produce the desired quality of semi-conductor.

All the materials, apparatus and procedures necessary to practice theinvention are available. A suitable procedure follows. A silicon blank,such as a material prepared by pulling ribbon or rod from a melt, issliced, polished and placed in a deposition chamber. A layer of silicondioxide having a thickness of at least 2000 Angstroms to 10,000Angstroms, usually from 6000 to 10,000 Angstroms, is grown on thesurface by feeding oxygen into the chamber. A layer of appropriatephotoresist material is then applied and dried. The layer of photoresistis exposed through a mask, developed, rinsed and cured. The photoresistcan be a positive or negative photoresist. The uncured areas are removedwith solvent to expose portions of the silicon oxide layer. The layer isetched with a mixture of 10% hydrofluoric acid, or other suitablesolvent to expose portions of the silicon surface. The remainingportions of the photoresist are removed with suitable solvent, such assulfuric acid-hydrogen peroxide. The silicon oxide masked blank is thenplaced in a deposition apparatus and a silicon layer is grown byepitaxial growth process and proceeds from the openings and across thebridging portions of the silicon dioxide mask to form a continuous layerwith less defects and fewer impurities.

It is to be understood that only preferred embodiments of the inventionhave been described and that numerous substitutions, modifications andalterations are permissible without departing from the spirit and scopeof the invention as defined in the following claims.

We claim:
 1. A semiconductor article comprising:a blank of semiconductorhaving a first level of impurities; a film of a first mask having apattern of openings applied to a surface of the blank and said filmbeing impervious to diffusion of said impurities from said semiconductorsurface; a first continuous layer of grown semiconductor on said maskextending from said openings over the bridges of mask between theopenings to form a first continuous layer of semiconductor having alesser level of impurities and defects than said blank, and having lessdefects and impurities in areas overlying said bridges than areasoverlying said openings; a second film of mask impervious to diffusionof said impurities having a pattern of openings in registration with thebridges of the first mask and a pattern of bridges overlying theopenings in the first mask; and a second continuous layer ofsemiconductor grown on said second mask having a lower impurity anddefect level than the first layer, said second layer extending from saidopenings and over the bridges of the second mask to form said secondcontinuous layer.
 2. An article according to claim 1 in which theopenings comprise 50% to 95% of the area of said surface.
 3. An articleaccording to claim 2 further including at least one further set of afilm of mask with a pattern of openings and a continuous layer ofsemiconductor grown thereon applied to the second continuous layersemiconductor.
 4. An article according to claim 3 in which the patternof the additional mask does not register with the pattern of the secondmask.
 5. An article according to claim 4 in which the semiconductor issilicon.
 6. An article according to claim 5 in which the mask materialis silicon dioxide.